diff --git a/R/data.table.R b/R/data.table.R
index 75d7f55fb..a0d781978 100644
--- a/R/data.table.R
+++ b/R/data.table.R
@@ -3238,10 +3238,7 @@ is_constantish = function(q, check_singleton=FALSE) {
     if (verbose) {catf("Creating new index '%s'\n", paste(idxCols, collapse = "__"));flush.console()}
     if (verbose) {last.started.at=proc.time();catf("Creating index %s done in ...", paste(idxCols, collapse = "__"));flush.console()}
     idx = forderv(x, idxCols, sort=TRUE, retGrp=FALSE, lazy=TRUE)
-    if (!isTRUE(getOption("datatable.forder.auto.index"))) { ## forder can write index, but disabled for now, see #4386
-      if (is.null(attr(x, "index", exact=TRUE))) setattr(x, "index", integer())
-      setattr(attr(x, "index", exact=TRUE), paste0("__", idxCols, collapse=""), idx)
-    }
+    maybe_reset_index(x, idxCols, idx) ## forder can write index, but disabled for now, see #4386
     if (verbose) {cat(timetaken(last.started.at),"\n");flush.console()}
     if (verbose) {catf("Optimized subsetting with index '%s'\n", paste(idxCols, collapse = "__"));flush.console()}
   }
diff --git a/R/setkey.R b/R/setkey.R
index d4ec9e3b7..c7ee8b0bf 100644
--- a/R/setkey.R
+++ b/R/setkey.R
@@ -72,10 +72,7 @@ setkeyv = function(x, cols, verbose=getOption("datatable.verbose"), physical=TRU
     o = forderv(x, cols, sort=TRUE, retGrp=!physical, lazy=TRUE)
   }
   if (!physical) { # index COULD BE saved from C forderMaybePresorted already, but disabled for now
-    if (!isTRUE(getOption("datatable.forder.auto.index"))) {
-      if (is.null(attr(x, "index", exact=TRUE))) setattr(x, "index", integer())
-      setattr(attr(x, "index", exact=TRUE), paste0("__", cols, collapse=""), o)
-    }
+    maybe_reset_index(x, cols, o)
     return(invisible(x))
   }
   if (length(o)) {
@@ -143,6 +140,13 @@ is.sorted = function(x, by=NULL) {
   # Return value of TRUE/FALSE is relied on in [.data.table quite a bit on vectors. Simple. Stick with that (rather than -1/0/+1)
 }
 
+maybe_reset_index = function(x, idx, cols) {
+  if (isTRUE(getOption("datatable.forder.auto.index"))) return(invisible())
+  if (is.null(attr(x, "index", exact=TRUE))) setattr(x, "index", integer())
+  setattr(attr(x, "index", exact=TRUE), paste0("__", cols, collapse=""), idx)
+  invisible(x)
+}
+
 ORDERING_TYPES = c('logical', 'integer', 'double', 'complex', 'character')
 forderv = function(x, by=seq_along(x), retGrp=FALSE, retStats=retGrp, sort=TRUE, order=1L, na.last=FALSE, lazy=getOption("datatable.forder.lazy",NA)) {
   if (is.atomic(x) || is.null(x)) {  # including forderv(NULL) which returns error consistent with base::order(NULL),